Method for forming a fiber glass racket frame



e 1969 R. w. ESHBAUGH 3,483,055

METHOD FOR FORMING A FIBER GLASS RACKET FRAME Filed March 28, 1966 5Sheets-Sheet 1 INVENTOR Faber-7 W- fsfi'bauyh ATTORNEYS Dec. 9, 1969 w.ESHBAUGH 3,483,955

METHOD FOR FORMING A FIBER GLASS RACKET FRAME Filed March 28, 1966 5Sheets-Sheet 2 ORNEYS Des. 9, 1969 R. w. ESHBAUGH 3,433,055

METHOD FOR FORMING A FIBER GLASS RACKET FRAME Filed March 28, 1966 5Sheets-Sheet 5 4 5 6' 019651?! T PAPLB MET/u Fa/L INVENTOR Pober-i N-Eshbauyh,

ORNEYs Dec 9, 196 R. w. ESHBAUGH METHOD FOR FORMING A FIBER GLASS RACKETFRAME 5 Sheets-Sheet 4 Filed March 28, 1966 INV EN TOR RoberL w. Eshboi;9 h

A ORNEY' Dec. 1969 R. w. ESHBAUGH METHOD FOR FORMING A FIBER GLASSRACKET FRAME 5 Sheets-Sheet 5 Filed March 28, 1966 INVENTOR Robe 14.55%

iZW A9 ATTORNEYS United States Patent U.S. Cl. 156189 25 Claims ABSTRACTOF THE DISCLOSURE A method for producing racket frames (tennis, etc.)essentially from only flexible windable materials involving the windingof same about a suitable form defining means and then heat curing saidmaterials to a rigid condition conforming to the desired frameconfiguration.

This invention relates to a fiber glass racket frame and, moreparticularly, to a fiber glass racket frame which enables the user tocontrol the flight of a tennis ball or the like with a maximumuniformity and minimum effort for a long period of time.

It has been the custom, when providing a means to propel a ball in thegames of tennis, badminton, squash, and so forth, to utilize a racketframe of wood, the head of which is provided with a nylon or gut stringwhich contacts the ball to propel it. However, these wooden racketssuffer from many disadvantages, which include relatively low tensile andcompressive strengths, susceptibility to warpage, scufiing, and thelike. Furthermore, these wooden rackets tend to lose their color and,therefore, attractiveness, over a period of years along with theirflexible properties, resulting in a relatively short fatigue life. Inaddition, these wooden frames must be of a certain relatively largecross section to provide the necessary strength, resulting in arelatively high wind resistance. Also in wooden frames, it is extremelydiflicult to manufacture a uniform product in regard to weight, balance,density and other mechanical properties since wood is a heterogeneousmaterial, varying in density, moisture content, and modulus ofelasticity.

Only recently have attempts been made to provide a racket frame of agenerally fiber glass material which, in general, has a higher strengthand a greater freedom from warpage, etc. However, these frames have alsosuffered in several respects, including a relatively high cost and lackof product uniformity, these being due to the fact that there has notyet been proposed any type of apparatus and method of uniformlymanufacturing these rackets which result in a relatively eflicientutilization of materials in order that the cost of each individualracket can be reduced to a competitive level with those of the woodenrackets. Nor has there yet been proposed a rapid and simple fabricationtechnique for the above-mentioned fiber glass racket, which provides anefficient utilization of materials.

It is, therefore, an object of this invention to provide a fiber glassracket frame which exhibits a greater strength than the wooden racketframes; a greater fatigue life; a

more advantageous control of degree of flexibility and stiffness; agreater resistance to damage, color and fading; a greater resistance towarpage, moisture, flame and chemicals.

It is a further object of this invention to provide a distinctimprovement over known fiber glass designs by providing an apparatus anda rapid, simple fabrication tech- 3,483,055 Patented Dec. 9, 1969 icenique which permits a more efficient utilization of materials, thusresulting in a racket frame which is more economical and easier toproduce.

Briefly summarized, the present invention features a racket frame formedby a mold which consists of several component parts, including a frontand back plate, a pair of clamping members, a pair of flanged arcuatearms, and a mandrel shaped according to the desired shape of thefinished racket frame. In the manufacture of the frame of the presentinvention, an inner loop or ring of uncured, substantially fiber glass,heat curable, flexible material is wound or placed over the mandrelbetween the two side plates and an outer loop or ring of the samematerial as above, but of a considerably larger diameter is wound orplaced over the inner material in an abutting relation, the lowerportion of the outer material hanging down to the bottom of the mold. Apair of clamping members are then clamped over this lower portion of theouter material to form the frame handle and the mold itself is securedtogether. The inner material together with the upper portion of theouter material forms the frame head, and the flanged arcuate arms maythen be pivoted downwardly over this head to form a groove along thehead to aid in the subsequent stringing of the frame. The materials arethen cured in the mold to form a. single racket frame which is thensubjected to several finishing treatments.

According to another embodiment of the invention, an elongated stockpiece which is substantially thicker than the normal width of theaverage racket is formed and subsequently sliced into a plurality ofindividual rackets. The apparatus for forming this stock piece includesan elongated lower mandrel about which an inner loop of substantiallyfiber glass heat curable, flexible material of considerable width iswound or placed, preferably in a helical manner. Also provided is a rodextending parallel to the lower mandrel; and an outer loop of a similarmaterial is wound or placed around the rod and mandrel with the upperportion of each material being in an abutting relationship over theupper portion of the mandrel. The rod is then moved in a direction awayfrom the lower mandrel to stretch the lower portion of the outermaterial. A pair of clamping members are then clamped between the rodand the mandrel which forms a portion of the outer material into theframe handle. A pair of arms substantially similar to those discussedabove may then be utilized to form the grooves in the racket heads. Inthis embodiment the stock piece is removed from the mold after curingand then sliced in a direction so that a plurality of individual framesare formed.

Reference is now made to the accompanying drawings for a betterunderstanding of the nature and objects of the present invention, whichdrawings illustrate the best mode presently contemplated for carryingout the objects of the invention and its principles and are not to beconstrued as restrictions or limitations on its scope.

FIG. 1 is a plan view of the major components for forming a singleracket frame according to the subject invention;

FIG. 2 is a plan view illustrating the preliminary step in theassembling of the mold and the forming of the racket frame;

FIG. 3 is a plan view with the mold assembled, but with the front plateomitted for the sake of clarity;

FIG. 4 is a side elevation of the assembled mold;

FIGS. 5 and 6 are sectional views taken along lines 55 and 6-6respectively of FIG. 3;

FIG. 7 is a sectional view of a typical ply of the racket frame takenalong line 7-7 of FIG. 2;

FIG. 8 is aperspective view of an apparatus used in the forming of astock piece from which' a plurality of rackets may be formed, showingthe inner ring which forms a portion of the racket head wrapped about alower mandrel;

FIG. 9 is a fragmentary perspective view of the device of FIG. 8 showinga further step in the assembly of a stock piece from which a pluralityof frames may be formed;

FIG. 10 is a perspective view of a device for ejecting the finishedstock piece from the apparatus of FIGS. 8 and 9;

FIG. 11 is a perspective view showing the arrangement of FIGS. 8 and 9during the step of curing the stock piece;

FIG. 12 is a side elevation of the stock piece being ejected from themachine;

FIG. 13 is a side elevation of a completed stock piece with phantomlines indicating lines through which the stock piece may be cut to formseparate racket frames.

Referring now to the drawings in general and specifically to FIGS. 14thereof, there is shown, in general, a mold 10 which is formed ofvarious component parts, including a back plate 12, an elliptical shapedmandrel 16 and a front plate 14. Also provided are a pair of clampingmembers 18 and 20 and a center piece 22 which does not form a portion ofthe mold, but rather is included in the final cured racket frame. Afterthe materials to be cured are placed in the mold in a manner to bediscussed in detail later, the mold is tightened, the means foreffecting this being seen by a reference to FIG. 4. As shown, back plate12 and front plate 14 each have a plurality of holes 24 through which isinserted bolts 26. Mandrel 16, which is inserted between the plates,also is provided with holes 24 through which the bolts 26 extend. Nuts28 are then tightened over the bolts 26 in order to lock the partstogether.

Clamping members 18 and 20 are provided to form the frame handle and areseen in a disassembled condition in FIG. 1 and, as shown in FIG. 3, aresecured to the side plates 12 and 14 by means of a pair of pins 30extending outwardly from the lower portion of each of said clampingmembers. Each pin engages in a corresponding L-shaped slot 32 formed oneach side of back plate 12 and front plate 14, the L-shaped slot beingprovided to lock the clamping members in this operative position as seenin FIG. 3. To secure the upper portion of each clamping member to theside plates, there is provided a securing means shown generally at 34 inFIGS. 1, 3 and 4 and more particularly in FIG. 6. As seen in FIG. 6,these clamp securing means 34 comprise a plate 36 extending through aportion of each clamping member and abutting the side edges of each ofsaid front and back plates 12 and 14 respectively. Plates 36 are securedto the mold by means of bolts 38 which extend through the plates 36 andare threaded into the back and front plates 12 and 14 as shown in FIG.6.

In order to provide a groove extending around the outer surface of thefinished frame head 45, there may be provided, on the mold, a pair ofarcuate shaped arms 40 .and 42 which may be pivotally connected toplates 12 and 14 as seen in FIG. 2 by means of a hinge 44. In theiroperative position these arcuate arms extend substantially around thehead portion 45 of the racket frame and are secured to the clampingmembers 18 and 20 respectively by means of a bolt clamp shown generallyat 46 in FIG. 3. Each of the bolt clamps extending through therespective arcuate arms are threaded into a hole formed in the clampingmembers 18 and 20. As better seen in FIG. 5, a flange 41 is provided onthis arm 40 which engages the material to be cured as shown to form agroove in the outer surface of the cured frame. It is to be understoodthat a similar flange is provided on arm 42.

Finally a center piece 22 is provided which is of a plastic, plasticfoam, or light wood and which serves as aspacer and filler in that itretainsthe limp uncured composite rings in proper position in the moldand provides a means against which clamping members 18 and 20 may act.

The material used in forming the racket frame of the present inventionis better seen in FIG. 2 which shows the mold in a partially assembledstate. This material includes an inner loop or ring 50 extending aroundthe mandrel 16 which may have its ends cut and overlapped near thebottom end of the mandrel as shown; and an outer loop or ring 52 whichis of a considerably larger diameter than the inner loop so that whenthe outer loop is placed over the inner loop as shown, the lower portionof the outer loop will extend downwardly through the entire length ofthe mold to form the handle portion 47 of the racket frame. The ringsmay be individually wound over the mandrel or manually placed thereonafter being formed on a separate mandrel.

FIGS. 5, 6 and '7 all show portions of the cured material in across-sectional view and FIG. 7 identifies each component of thematerial before it is cured, it being understood that it is within thescope of the invention to vary the components as desired. Since FIG. 5is taken along a portion of the cured frame head 45, the structure ofthe inner loop 50 and the outer loop 52 will be molded together while,since FIG. 6' depicts a cross section of the frame handle 47, the outerloop 52 is shown between the sides of which a center piece 22 eX- tends.It should be emphasized that each individual layer of material subtendsthe width of the racket and extends from the bottom portion of the framehandle upward around the frame head and then downward back towards thehandle.

The inner loop and outer loop may each be separately formed by applyinga thermos-setting adhesive to a plurality of glass rovings and windingthe rovings onto the mandrel 16 or onto a separate mandrel along withabsorbent paper and foil, the weight of the frame being, of course,determined by the number of wraps or plies present in each ring. If aseparate mandrel is used, the limp uncured loops which are thus formedare then placed on the mandrel 16 as discussed above.

The thermo-setting adhesive may be in the form of an epoxy or polyesterresin blended with an appropriate amount of a suitable type of curingagent. The absorbent paper is included to absorb this adhesive and thusprovide an even distribution thereof throughout the frame and may be inthe form of stands, tapes, or sheets of cellulose, asbestos or plasticfoam. The foil is utilized to act as a barrier between adjacent layersof the glass and paper as well as to provide added strength to thefinished frame. In the event added strength is not deemed necessary,plastic tape or paper tape may be utilized in place of the foil.

It is to be understood that variations of the type and amount of thesematerials may be made within the scope of the invention. For example, ithas been found that it is highly desirable to use a glass cloth in placeof the glass rovings in the outer surface of the racket since the formerprovides added shear strength in the transverse direction of the frame.

In the steps of forming the racket frame according to the presentinvention, the mandrel 16 may be initially coated with a release agentand an uncured or B-staged" loop of the above mentioned materialsdirectly wound or manually placed around the mandrel. The free ends ofthis loop may then be cut and overlapped adjacent the lower portion ofthe mandrel as seen in FIG. 2 in order to provide additional materialfor the throat portion of the frame, or the loop may be left on themandrel in its original form. Then the outer loop 52 is either directlywound or manually placed over the inner loop 50 on the mandrel 60, thisouter loop being of a larger diameter than the inner loop and of thesame materials. Side plates 12 and 14, which may be also coated with arelease agent, are then secured adjacent each face of the mandrel bymeans of the bolts 26 as discussed above and the side plates are thentightened to the desired degree and locked by tensioning nut 28 on thebolts. The outer loop is then stretched downwardly by means not shown sothat the lower portion thereof extends at least to the lower end of themold which removes wrinkles, air or resin pockets from the material andminimizes slippage. An elongated spacer 22 may then be inserted withinthe lower portion of this outer loop and adjacent the lower portion ofthe mandrel as seen in FIG. 2.

The lower portion of the stretched outer loops 52 is then clamped inwardby the clamping members 18 and 20 so that it will abut the center piece,thus densifying the laminate by squeezing out the excess resin whileforming the frame handle. It should be understood that the stretchingmeans discussed above must give slightly in a direction towards themandrel against the action of the clamping members while stillmaintaining the requisite degree of tension. Arms 40 and 42 which arehinged between the side plates, may then be pivoted downwardly andclamped by means of bolts 46 to the upper p0rtion of the clampingmembers 18 and 20, the flange 41 on each arm extending into the outersurface of the outer loop in order to form a groove therein which isutilized to protect the racket strings from scuffing.

The materials are then cured by any known method, such as by use of anelectrical heating element or steam pipes attached directly to the moldplates or by simply placing the mold inside an oven. The time,temperature and conditions necessary to polymerize the material dependon its chemical nature, it being understood that production and economyconsiderations dictate the choice of a fast curing system with a heatcure. For example, it has been found that a typical initial curing timefor a thermo-setting adhesive such as epoxy resin blended with an aminecuring agent is for one hour at 200 F.

The cured racket frame is then removed from the mold by simplydisassembling the mold in reverse order as discussed earlier andknocking out the mandrel from within the frame head. The frame then maybe subjected to an additional heat cycle to insure completepolymerization. For a typical system it could be post cured at 250275 F.for two hours. This may be performed either with the frame on or off themold, the latter being more economical as it permits resue of the moldearlier in the production cycle.

A series of final finishing steps are then performed on the cured racketframe which may consist of removing any fiash present, cutting the shaftto the proper length, drilling the string holes, adding any necessarylabels and decorative decals, buffing and polishing, forming any type ofhandle, and spraying a glossy sealer varnish or the like over the frame.

The string holes are drilled within the groove formed in the outersurface of the frame head, so that the string will not extend outwardfrom the outer surface of the frame head, the string thus being lesssusceptible to being frayed in actual use.

Referring now to FIG. 813 which relate to another embodiment of thepresent invention, an elongated stock piece is formed which has a crosssection corresponding to the shape of the finished racket frame. Thisstock piece, of course, is considerably wider than the finished frameand after it has been molded it is sliced into a plurality of individualframes. The apparatus for forming the frame stock piece includes a base60 in which is jOurnaled a lower mandrel 62, means being provided inbase 60 for rotating mandrel 62. As in the first embodiment the mandrel62 may be shaped according to the frame head, and is preferablyelliptical. As seen in FIG. 8 tension rod 64, the longitudinal axis ofwhich is parallel to that of mandrel 62, is mounted in base .60 at 66 ina manner so that it can move in a direction toward and away from themandrel 62, for reasons that will be explained later,

it being understood that any known means are provided in base member 60for permitting this movement.

The stock piece is formed by winding or placing an inner layer 61 ofsubstantially fiber glass material directly around the mandrel 62. Thismaterial may be of the same composition as set forth in the firstembodiment with one exception. The glass rovings may be helically woundaround the mandrel to provide a shear strength in all directions andthus eliminating the need for glass cloth or the like, which is adesirable component in the forming of a single frame, as describedearlier. The helical windings are shown by the lines 93 and 95 in FIG.13, it being understood that the absorbent paper and foil may be also bewound similarly, or applied as a sheet extending the entire width of themandrel.

After the inner loop has been wound around mandrel 62, an outer loop orring 63, which may be of the same material as the outer loop of thesingle frame, is placed around the mandrel and the tension rod 64, theouter loop, of course, extending over a portion of the inner loop asshown in FIG. 9. As in the first embodiment, each loop may be directlywound around the mandrel and rod or formed on a separate mandrel andthen manually placed over mandrel 62 and rod 64. The tension rod is thenmoved in a direction away from the mandrel in order to place a predetermined tension on the outer loop 63. After center piece 68 has beeninserted Within the outer loop 63 and adjacent mandrel 62, a pair ofclamping members 77 and 79 are clamped adjacent each side of the outerloop 63 as shown in FIG. 11 to form the frame handle. Rams or the likemay be provided to exert an inward clamping force on members 77 and 79,as shown in FIG. 11, it being understood that rod 64 will have to give alittle to accommodate this inwardly directed force. Infrared heat lamps,or the like, along with electrical heating elements 82, which extendthrough clamping members 77 and 79, may be utilized to effect the curingof the stock piece.

It should be understood that means may also be provided to form a seriesof grooves in the stock piece before it is cured, which correspond tothe grooves which protect the strings, as discussed in relation to thefirst embodiment.

Means are also provided to eject the cured stock piece from the mandreland tension rod after the clamping members 77 and 79 have beenretracted, this means being shown in FIGS. 10 and 12. A pair ofcylinders 70 and 72 are provided which are mounted in base member 60 andwhich have a pair of piston rods 74 and 76, respectively slidablymounted therein. Attached to the free end of the piston rods 74 and 76is an elliptical yoke shown at 78 which, in the assembled conditionshown in FIG. 12, extends around the mandrel 62. When the finished stockpiece has been sufiiciently cured, it is ejected from the mandrel andtensioning rod by action of the pistons 74 and 76. The pistons may beactuated by means of a fluid or the like being forced into the cylinders70 and 72 in any known manner.

The temperature and time conditions used in the curing of the. inner andouter loops may be the same as discussed earlier.

It is to be understood that the curing operation is nor- {nallyperformed while the inner and outer loops are still in the mold and thatthe cured stock piece also may be sub ected to a post curing attemperatures which correspond to those in the first embodiment, saidpost curmg occurring before or after the ejection of the stock piecefrom the mold. Then the. stock piece is sliced along lines 92 in FIG. 12to form a plurality of individual frames which, of course, then may besubjected to finishing operations as discussed in relation to thefirst-mentioned embodiment.

It is within the scope of the instant disclosure to add color pigmentsto the resin for the different layers of the racket frame. The differentcolored layers so formed may be separated by a substantially impermeablematerial such as foil, paper or tape, as discussed above. For example, a

7 black pigment could be added to the resin binding the materials to theleft of the foil in FIG. 7, and a red pigment added to the material tothe right.

The racket frames formed by the method and apparatus of the presentinvention have many advantages. For example, they will exhibit tensileand compressive strengths in excess of 80,000 lbs. per square inch,which is over three times the strength of a typical laminated woodenracket. Also, a greater fatigue life is present in the racket frame ofthe present invention, since the stress level during use is a smallerpercentage of the ultimate strength than that of a wooden frame,resulting in a greater reserve strength in the frame of the presentinvention. Another advantage of the frame of the present invention isthat it can be given a controlled degree of flexibility and stiffnessthrough a selection of several parameters, such as type of material,amount of material, degree of curing, etc. Furthermore, the surface ofthe. fiber glass racket of the present invention is much harder thanthat of wooden rackets and is, therefore, more scratch and damageresistant. Also, the racket of the present invention maintains itsoriginal permanent color, and will not warp because it is inert.

Of course variations of the specific construction and arrangement ofthis type of apparatus and method herein disclosed may be made by thoseskilled in the art without departing from the invention.

What is claimed is:

1. A method of manufacturing a plurality of racket frames comprising thesteps of placing a first loop of a combination of heat curable flexiblematerials around an elongated mandrel, positioning an adjustabletensioning rod in a spaced position from said mandrel so that theirrespective longitudinal axes are parallel, placing a second loop ofsimilar type materials around said mandrel and said rod so that aportion of said second loop abuts a portion of said first loop, movingsaid rod in a direction away from said mandrel until a predeterminedtension is placed on said second loop, clamping another portion of saidsecond loop inward to form the racket handles, curing said material,removing the cured material from said mold, and slicing said curedmaterial into a plurality of individual racket frames.

2. The method of claim 1 further comprising the step of coating saidmandrel and said rod with a release agent before said materials areplaced thereon.

3. The method of claim 1 further comprising the step of inserting aspacing means within said second loop between said rod and said mandrelbefore said clamping step.

4. The method of claim 1 further comprising the step of adding a colorpigment to the adhesive of at least one of said loops before said curingstep.

5. The method of claim 1 further comprising the step of finishing saidframes after said slicing step.

6. The method of claim 1 wherein at least one of said loops is formed bythe steps of applying a thermo-setting adhesive to a plurality of glassrovings and winding same along with an absorbent material and foil ontosaid mandrel at predetermined intervals.

7. The method of claim 6 wherein at least said rovings are helicallywound on said mandrel.

8. The method of claim 1 wherein at least one of said loops is formed bythe steps of applying a thermo-setting adhesive to a plurality of glassrovings and winding same along with an absorbent material and foil ontoa separate mandrel at predetermined intervals before placing the formedring around said first mentioned mandrel.

9. The method of claim 8 wherein at least said rovings are helicallywound on said separate mandrel.

10. A method of manufacturing a racket frame consisting of a head andhandle portion, comprising the steps of:

(a) laying an inner layer of flexible, windable material which is heatcurable to a rigid condition contiguously along at least a portion of acontinuous surface which defines the desired head shape of a racketframe;

(b) laying an outer layer of flexible, windable material, which is heatcurable to a rigid condition, between two spaced apart points along aportion of the longitudinal extent of said inner layer which iscontiguous with said surface, and convergingly extending straight spacedapart sides of said outer layer from said points towards a point alongthe desired axis of the frame handle;

(c) pressing the converging straight sides towards each other to contactthem along a remaining portion of said inner layer and to conform themto the desired frame handle configuration;

(d) curing said layers to a rigid condition.

11. The method of claim 10, wherein said inner layer of material iswrapped around said continuous surface to form a closed curvilinear looptherearound, and wherein said straight sides extend tangentially fromrespective points along the curvilinear extent of said closed loop.

12. The method of claim 10, further including the step of inserting aspacer element between said converging sides and then pressing saidsides against said spacer element and curing said layers so as topermanently embed said spacer element between said sides.

13. The method of claim 10, further including prior to the curing step,the step of forming a groove along the extent of the outer peripheralsurface of said outer layer which conforms to the frame head portion.

14. The method of claim 10, wherein said inner layer is wrapped aroundsaid continuous surface to form a closed loop therearound.

15. The method of claim 14, wherein said inner layer is made longer thansaid continuous surface and the ends of said inner layer arecircumferentially overlapped to form said closed loop.

16. The method of claim 10, wherein said outer layer sides are made toconverge and meet up to and at a point corresponding to the end of thedesired frame handle and wherein said sides are tensioned to a wrinklefree condition while being pressed towards each other.

17. The method of claim 16, including the step of applying a pigment ofa different color to respective ones of the heat curable agents whichimpregnate respective ones of said strata of impregnated material.

18. The method of claim 10, wherein said outer layer is itself a closedcontinuous loop of material which is conformed to a portion of saidinner layer and extended therefrom as said converging straight sides toform a corner along the desired axis of the frame handle.

19. The method of claim 18, including the step of gripping said cornerand tensioning said sides to a wrinklefree condition while pressing saidsides towards each other.

20. The method of claim 10, wherein at least one of said layers isformed by laying a first strata of absorbent material over a strata of amaterial impregnated with a heat-curable agent and by laying a secondstrata of such impregnated material over said absorbent strata.

21. The method of claim 20, wherein said impregnated material comprisesa plurality of glass rovings impregnated with a thermosetting agent,which rovings are wound together with said absorbent material separatelyfrom said surface into the form of a ring which is then laid around saidsurface to form said one layer.

22. The method of claim 20, wherein a strata of metal foil is laid alongsaid second strata of impregnated material and thereupon the stratasequence of claim 20 is repeated.

23. The method of claim 22, which includes the step of applying apigment of a different color to respective ones of the heat curableagents which impregnate respective ones of said strata of impregnatedmaterial.

24. The method of claim 20, wherein said impregnated material is a glassroving.

25. The method of claim 24, wherein said roving is in the form of aglass cloth.

References Cited UNITED STATES PATENTS Boggs 156-171 10 FOREIGN PATENTS660,678 11/1951 Great Britain. 785,995 5/1935 France. 817,405 7/ 1959Great Britain.

HAROLD ANSHER, Primary Examiner J. C. GIL, Assistant Examiner U.S. Cl.X.R.

